On_delete options (CASCADE, PROTECT, SET_NULL) in Django - Step-by-Step Execution

Concept Flow - On_delete options (CASCADE, PROTECT, SET_NULL)

Delete Parent Object

↓

Check on_delete option

↓

CASCADE

↓

End

When a parent object is deleted, Django checks the on_delete option to decide what happens to related child objects.

Execution Sample

Django

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

# Deleting an Author triggers on_delete behavior on Book

This code shows a Book linked to an Author. Deleting the Author triggers the on_delete rule on Book.

Execution Table

Step	Action	on_delete Option	Effect on Child Object	Result
1	Delete Author object	CASCADE	Delete all related Book objects	Books deleted automatically
2	Delete Author object	PROTECT	Prevent deletion if related Book exists	Error raised, Author not deleted
3	Delete Author object	SET_NULL	Set Book.author to NULL if allowed	Author deleted, Books remain with author=NULL
4	Delete Author object	Other (e.g. DO_NOTHING)	No automatic action on Book	Possible DB error if FK constraint violated
5	End	-	-	Process complete

💡 Process ends after applying the on_delete rule to child objects.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Author Exists	True	False (CASCADE)	True (PROTECT)	False (SET_NULL)	Depends on option
Book Count	5	0 (CASCADE)	5 (PROTECT)	5 (SET_NULL, author=NULL)	Depends on option
Book.author FK	Author ID	N/A (deleted)	Author ID	NULL	Depends on option

Key Moments - 3 Insights

Why does PROTECT prevent deleting the parent object?

PROTECT raises an error if child objects exist, stopping deletion. See execution_table row 2 where deletion is blocked.

What happens to child objects with SET_NULL if the FK does not allow null?

If FK disallows null, SET_NULL causes a database error. The child FK must allow null for SET_NULL to work, as shown in execution_table row 3.

Does CASCADE delete only direct children or deeper related objects too?

CASCADE deletes direct children and can cascade further if those children have on_delete CASCADE. The example shows direct children deleted at step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what happens to Book objects when Author is deleted with CASCADE?

A Books are deleted automatically

B Books remain unchanged

C Books' author field is set to NULL

D Deletion is blocked with an error

💡 Hint

See execution_table row 1 under 'Effect on Child Object'

At which step does deletion fail due to PROTECT?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Check execution_table row 2 where error is raised

If Book.author FK does not allow null, what happens with SET_NULL on delete?

A Books are deleted

B Author deletion is blocked

C Books' author field is set to NULL

D No effect, deletion proceeds

💡 Hint

See key_moments question 2 about FK nullability and SET_NULL

Concept Snapshot

Django on_delete options control child object behavior when parent is deleted:
- CASCADE: deletes child objects automatically
- PROTECT: blocks deletion if children exist
- SET_NULL: sets FK to NULL if allowed
Choose based on desired data integrity and app logic.

Full Transcript

In Django, when you delete a parent object linked by a ForeignKey, the on_delete option decides what happens to related child objects. CASCADE deletes all related children automatically. PROTECT stops deletion and raises an error if children exist. SET_NULL sets the child's foreign key to NULL if the field allows nulls. Other options exist but these three are common. This visual shows step-by-step what happens when deleting a parent with each option, how variables like object existence and foreign keys change, and common beginner questions about behavior and errors. Understanding these helps keep your database consistent and avoid unexpected deletions or errors.

Practice

1. What does the on_delete=models.CASCADE option do in Django models?

easy

A. Deletes related objects automatically when the referenced object is deleted.

B. Prevents deletion of the referenced object if related objects exist.

C. Sets the related field to null instead of deleting the object.

D. Raises an error if the related object is missing.

Solution

1. Step 1: Understand CASCADE behavior

   The CASCADE option means when the referenced object is deleted, all related objects are also deleted automatically.

2. Step 2: Compare with other options

   Unlike PROTECT or SET_NULL, CASCADE deletes dependent objects without error or nullifying fields.

3. Final Answer:

   Deletes related objects automatically when the referenced object is deleted. -> Option A

4. Quick Check:

   CASCADE = auto-delete related objects [OK]

Hint: CASCADE means delete all linked items automatically [OK]

Common Mistakes:

• Confusing CASCADE with PROTECT which blocks deletion
• Thinking CASCADE sets fields to null instead of deleting
• Assuming CASCADE raises errors on deletion

2. Which of the following is the correct syntax to use SET_NULL in a Django ForeignKey field?

easy

A. models.ForeignKey(OtherModel, on_delete=models.SET_NULL)

B. models.ForeignKey(OtherModel, on_delete=models.SET_NULL, null=True)

C. models.ForeignKey(OtherModel, on_delete=models.SET_NULL, blank=True)

D. models.ForeignKey(OtherModel, on_delete=models.SET_NULL, default=None)

Solution

1. Step 1: Recognize SET_NULL requires nullable field

   Using SET_NULL requires the field to allow null values, so null=True must be set.

2. Step 2: Check each option for null=True

   Only models.ForeignKey(OtherModel, on_delete=models.SET_NULL, null=True) includes null=True along with SET_NULL, making it valid syntax.

3. Final Answer:

   models.ForeignKey(OtherModel, on_delete=models.SET_NULL, null=True) -> Option B

4. Quick Check:

   SET_NULL needs null=True [OK]

Hint: SET_NULL needs null=True to allow null values [OK]

Common Mistakes:

• Omitting null=True with SET_NULL causes errors
• Confusing blank=True with null=True for database nulls
• Using default=None without null=True

3. Given the models:

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.PROTECT)
    title = models.CharField(max_length=100)

What happens if you try to delete an Author who has related Book entries?

medium

A. The author is deleted but books remain with invalid author references.

B. The author is deleted and all related books are deleted too.

C. The author is deleted and the book's author field is set to null.

D. The deletion is blocked and raises an error.

Solution

1. Step 1: Understand PROTECT behavior

   PROTECT prevents deletion of the referenced object if related objects exist, raising an error instead.

2. Step 2: Apply to given models

   Since Book has a ForeignKey with PROTECT to Author, deleting an Author with Books will raise a ProtectedError.

3. Final Answer:

   The deletion is blocked and raises an error. -> Option D

4. Quick Check:

   PROTECT blocks deletion if related objects exist [OK]

Hint: PROTECT stops deletion if related objects exist [OK]

Common Mistakes:

• Assuming PROTECT deletes related objects like CASCADE
• Thinking PROTECT sets fields to null
• Ignoring the error raised on deletion attempt

4. Consider this model definition:

class Comment(models.Model):
    post = models.ForeignKey(Post, on_delete=models.SET_NULL)
    content = models.TextField()

What is the error in this code?

medium

A. No error, the code is correct.

B. ForeignKey cannot use SET_NULL as on_delete option.

C. Missing null=True on the ForeignKey field.

D. TextField cannot be used with ForeignKey.

Solution

1. Step 1: Check requirements for SET_NULL

   Using SET_NULL requires the ForeignKey field to allow null values by setting null=True.

2. Step 2: Identify missing null=True

   The code does not include null=True on the post field, causing an error.

3. Final Answer:

   Missing null=True on the ForeignKey field. -> Option C

4. Quick Check:

   SET_NULL needs null=True [OK]

Hint: SET_NULL requires null=True on ForeignKey [OK]

Common Mistakes:

• Assuming SET_NULL works without null=True
• Confusing TextField usage with ForeignKey
• Thinking SET_NULL is invalid on ForeignKey

5. You have two models:

class Category(models.Model):
    name = models.CharField(max_length=50)

class Product(models.Model):
    category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True)
    name = models.CharField(max_length=100)

If a Category is deleted, what happens to the related Product entries and why is this setup useful?

hard

A. Products keep their category field as null; useful to retain products without category.

B. Products are deleted automatically; useful to keep database clean.

C. Deletion is blocked; useful to prevent accidental data loss.

D. Products keep old category id; useful for historical reference.

Solution

1. Step 1: Understand SET_NULL with null=True

   When the referenced Category is deleted, the Product's category field is set to null instead of deleting the Product.

2. Step 2: Explain usefulness

   This allows products to remain in the database without a category, which is useful if products should not be deleted just because their category is removed.

3. Final Answer:

   Products keep their category field as null; useful to retain products without category. -> Option A

4. Quick Check:

   SET_NULL + null=True keeps related objects, nullifies field [OK]

Hint: SET_NULL keeps objects, sets field null to keep data [OK]

Common Mistakes:

• Assuming related products get deleted with SET_NULL
• Thinking deletion is blocked like PROTECT
• Believing old category id remains after deletion